Optimization of memory usage by dirents during ZIM creation

[veloman-yunkan]

This is the easy part of the optimization addressing #1048.

Using the test flow from #1045:

def test_creator_many_redirects(fpath, lipsum):
    random.seed(123)
    with Creator(fpath) as c:
        for item_i in range(1000000):
            c.add_item(StaticItem(path=f"dedup{item_i}", content=lipsum, mimetype="text/html"))
            for redirect_i in range(10):
                to_index = random.randint(0, item_i)
                c.add_redirection(f"home{item_i}{redirect_i}", "", f"dedup{to_index}", {})

the performance figures changed as follows:

	High watermark memory usage (MB)	Runtime (seconds)
main branch	2241	661
This PR	1690	540

As you can see, there is improvement in speed too.

[codecov]

Codecov Report

❌ Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.33%. Comparing base (0f16c3c) to head (bc0fa61).

Files with missing lines	Patch %	Lines
src/writer/creator.cpp	58.33%	0 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1055      +/-   ##
==========================================
+ Coverage   56.26%   56.33%   +0.07%     
==========================================
  Files         101      101              
  Lines        5014     5013       -1     
  Branches     2186     2183       -3     
==========================================
+ Hits         2821     2824       +3     
  Misses        737      737              
+ Partials     1456     1452       -4     

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[veloman-yunkan]

@benoit74 @kelson42 Further optimization is possible and I plan to pursue it in any case. Should I include it in this PR or you want to be able to enjoy current improvements by merging the changes early?

[benoit74]

I'm not in a hurry to have anything merge on this matter. This is more a medium term (very important) improvement for me.

[kelson42]

@benoit74 @kelson42 Further optimization is possible and I plan to pursue it in any case. Should I include it in this PR or you want to be able to enjoy current improvements by merging the changes early?

Great to see this PR coming!

It is really up to you, but if you bundle these optims in one PR, this sounds optimal to me.